1. Field of the Invention
The present invention relates to a wavelength converter device comprising a structure having a plurality of cascaded optical resonators.
The present invention relates, moreover, to an use of a structure comprising a plurality of cascaded optical resonators for generating a radiation at frequency ωg through non-linear interaction between at least one pump radiation at frequency ωp and at least one signal radiation at frequency ωs.
Furthermore, the present invention relates to an use of a structure, comprising a plurality of cascaded optical resonators made of a non-linear material, for altering the optical spectrum of at least one signal radiation at frequency ωs by non-linear interaction within the material of the optical resonators.
Moreover, the present invention relates to an apparatus for an optical network node comprising a routing element and a wavelength converter device of the invention.
Additionally, the present invention relates to an optical communication line comprising an optical transmission path for transmitting at least one signal radiation at frequency ωs and a wavelength converter device of the invention.
2. Description of the Related Art
Structures made of a plurality of cascaded optical resonators are known.
For example, A. Melloni et al. (“Synthesis of direct-coupled-resonators bandpass filters for WDM systems”, Journal of Lightwave Technology, Vol. 20, No. 2, February 2002, pages 296-303) disclose a structure consisting of cascaded direct-coupled ring resonators or cascaded Fabry-Pérot resonators for use as a bandpass filter.
Furthermore, U.S. Pat. No. 5,311,605 discloses an optical device comprising a length of optical waveguide having incorporated therein an extended sequence of coupled single-resonator structures for use as an optical slow wave structure. This document states that the structure may also be designed to provide a desired filter characteristic, a dispersion such as to correct for undesirable dispersion in other components of an optical system or to provide pulse expansion or compression. The Applicant notes that no mention of use of non-linear interactions is made in this document.
In a WDM (wavelength division multiplexing) optical communication system/network, wavelength management and wavelength routing control between nodes of the system is crucial for preventing wavelength blocking and facilitating cross connecting. To this end wavelength converter devices able to shift a signal radiation from an optical channel to another are required.
Non-linear wavelength converter devices using a parametric process are known.
A parametric process is a process typical of materials having a non-linearity of the χ2 or χ3 type according to which electromagnetic radiation at predetermined frequencies that propagate in such materials interact with each other for generating electromagnetic radiation at different frequencies from those that have generated them.
For example, a parametric process is a process according to which a pump radiation at frequency ωp that propagates in a non-linear material, interacting with a signal radiation at frequency ωs, generates a radiation at frequency ωg.
Typical parametric processes are a difference frequency generation process, according to which ωg=ωp−ωs, a sum frequency generation process, according to which ωg=ωp+ωs, a second or third harmonic generation process, according to which ωg=2ωp or, respectively, ωg=3ωp and a degenerate four-wave mixing (FWM) process according to which ωg=2ωp−ωs or ωg=2ωp+ωs.
For example, one-dimensional-photonic-crystal multiresonator structures (also called photonic band-gap structures) have been proposed for wavelength conversion through a second harmonic generation parametric process.
A one-dimensional photonic crystal structure typically consists of a periodical alternation of two layers of material having different refractive indexes. The multiple reflections at the interfaces between the two layers at different refractive index generate constructive and destructive interference between the transmitted light and the reflected light, so that the propagation of electromagnetic waves in the photonic crystal structure is allowed in some intervals of frequencies (or wavelengths) and forbidden in other intervals. The layers typically have thicknesses a and b of λ/4n (quarter-wave layer) or λ/2n (half-wave layer), where λ is the operating wavelength and n the refractive index of the layer, so as to form a periodic quarter-wave, half-wave or mixed quarter-half-wave structure.
WO 99/52015 describes a second harmonic generator based on a periodic photonic crystal structure. The described structure comprises a plurality of layers of a first and a second material that periodically alternate, and has a band edge at the pump radiation frequency and a second transmission resonance near the band edge of the second order band gap at the generated second harmonic frequency. The layers have thicknesses a and b of λ/4n or λ/2n.
U.S. Pat. No. 6,002,522 discloses a structure having materials with different refractive indexes and periodically arranged to form a photonic band-gap structure. Furthermore, it discloses to set the period of two different materials having different refractive indexes (that is the thickness of a pair of two different materials) at nearly half the wavelength of light used. This document teaches that the structure can be used to manufacture a wavelength converter by second harmonic generation, if a second-order non-linear optical material is used, and an optical switch if a third-order non-linear optical material is used.
Wavelength converter devices using the four-wave mixing process are known.
EP 0 981 189 discloses a non-linear wavelength converter device comprising an optical waveguide doped with a rare earth element. An input optical signal and at least one pump light cause four wave mixing (FWM) to occur in the optical waveguide and the FWM causes a converted optical signal to be produced in the optical waveguide. The optical signal and the pump light are amplified in the optical waveguide thereby the four-wave mixing converted optical power is increased.
In order to increase the four-wave mixing converted optical power, also wavelength converter devices using the four-wave mixing process in a single optical resonator have been disclosed.
P. P. Absil et al. (“Wavelength conversion in GaAs micro-ring resonators”, Optics Letters, Vol. 25, No. 8, Apr. 15, 2000, pages 554-556) disclose a device comprising a single micro-ring resonator wherein a pump wave of frequency ωp and a signal wave of frequency ωs are launched into the ring at two different resonant frequencies. A new converted wave is generated by degenerate FWM at the frequency ωg=2ωp−ωs. The Authors states that non-linear interactions are enhanced in the resonator.
U.S. Pat. No. 5,243,610 disclose a device comprising an input for an input light signal, an optical source for generating a pump light signal, a non-linear optical medium for receiving the pump and input light signals and an output. The non-linear optical medium includes a Fabry-Perot type semiconductor laser and frequency converts the input light signal to generate an output light signal using non-degenerate four-wave mixing. In this document it is stated that the FWM may be generated at a relatively lower power due to an internal electric field enlarged by confining the pump light and input signal light into a resonator.
U.S. Pat. No. 5,550,671 discloses a device comprising an input for a signal radiation, an optical source for generating a pump radiation, a laser cavity and an output. The laser cavity is composed of a rare-earth doped fiber and is defined by a pair of fiber Bragg gratings. By propagating through the laser cavity the signal and pump radiation generate by four wave mixing a new converted signal of wavelength within 10% of the signal radiation wavelength. In this document it is stated that the device can be made in compact form with a cavity length as small as 100 m and can provide inverted signals at the same intensity as the input signals.
The Applicant notes that in the above mentioned devices with a single optical resonator, the four-wave mixing converted optical power (i.e. the optical power of the radiation generated by four-wave mixing) depends on the pump power, on the resonator physical length and on the power reflectivity of the reflectors forming the resonator.
For cost, availability and reliability reasons, the pump power should be kept as low as possible. Therefore, the resonator physical length and the power reflectivity should be kept as high as possible in order to achieve high converted optical power values.
However, in this regard the Applicant notes that also the frequency difference between two consecutive resonant frequencies (free spectral range or FSR) and the bandwidth B of an optical resonator depend on the physical length and on the power reflectivity. Furthermore, for use in a WDM optical communication system, the FSR and bandwidth B of the resonator should be set according to the WDM system requirements (e.g., the bandwidth B′ of the WDM optical signals and the wavelength spacing thereof which is typically selected according to ITU-T recommendations).
It follows that, the physical length and the power reflectivity of the optical resonator should be selected according to the WDM optical communication system requirements and cannot be freely set to any desired value.
Accordingly, the Applicant notes that external factors may not allow desired values of the FWM converted optical power to be achieved. Thus, the above mentioned devices, using the four-wave mixing process in a single optical resonator, are not versatile.
The Applicant has thus faced the technical problem of providing an efficient and versatile wavelength converter device, capable of achieving high converted optical power values and, at the same time, meeting WDM optical communication system requirements.